Zirconium tetrachloride distillation

Molten Salt Distillation. Hafnium tetrachloride is slightly more volatile than zirconium tetrachloride, but a separation process based on this volatility difference is impractical at atmospheric pressures because only soHd and vapor phases exist. The triple point for these systems is at about 2.7 MPa (400 psia) and 400°C so that separation of the Hquids by distillation would necessarily require a massive pressurized system (13). 

The product gases are first cooled below 200°C to selectively condense so-called zirconium tetrachloride snow in a large space condenser. The sihcon tetrachloride subsequendy is condensed in a quench condenser wherein the warm gases are countercurrendy scmbbed with Hquid siUcon tetrachloride at —20° C. The siUcon tetrachloride is purified by stripping and distillation. 

Pure zirconium tetrachloride is obtained by the fractional distillation of the anhydrous tetrachlorides in a high pressure system (58). Commercial operation of the fractional distillation process in a batch mode was proposed by Ishizuka Research Institute (59). The mixed tetrachlorides are heated above 437°C, the triple point of zirconium tetrachloride. AH of the hafnium tetrachloride and some of the zirconium tetrachloride are distiUed, leaving pure zirconium tetrachloride. The innovative aspect of this operation is the use of a double-sheU reactor. The autogenous pressure of 3—4.5 MPa (30—45 atm) inside the heated reactor is balanced by the nitrogen pressure contained in the cold outer reactor (60). However, previous evaluation in the former USSR of the binary distiUation process (61) has cast doubt on the feasibHity of also producing zirconium-free hafnium tetrachloride by this method because of the limited range of operating temperature imposed by the smaH difference in temperature between the triple point, 433°C, and critical temperature, 453°C, a hafnium tetrachloride. 

Dichlorotoluene (2,4-dichloro-l-methylben2ene) constitutes 80—85% of the dichlorotoluene fraction obtained in the chlorination of PCT with antimony trichloride (76) or zirconium tetrachloride (77) catalysts. It is separated from 3,4-dichlorotoluene (l,2-dichloro-4-methylben2ene), the principal contaminant, by distillation. Chlorination of OCT with sulfuryl chloride gives mainly 2,4-dichlorotoluene and small amounts of the 2,3 isomer (78). 

The volatilities of both zirconium tetrachloride and hafnium tetrachloride are very similar to each other at normal operating temperatures, and their separation by a simple distillation or fractional distillation operation is not viable. However, when the mixed chloride vapor is contacted with an eutectic molten salt mixture of aluminum chloride and potassium chloride, zirconium chloride is preferentially absorbed. The vapor pressure difference between zirconium and hafnium tetrachlorides is greatly enhanced over the molten... 

Zirconium tetrachloride (2.33 g. 0.01 mol) is added to 40 ml. of carbon tetrachloride in a 100-ml. one-necked round-bottomed flask fitted with a water-cooled reflux condenser. Freshly distilled 1,1,1-trifluoroacetylacetone (1,1,... 

Tetramethylzirconium was prepared (59) by the reaction of stoichiometric amounts of methyllithium and zirconium tetrachloride in an ether-toluene solvent mixture at —45°C. Methylation is accompanied by the appearance of a yellow color and precipitation of lithium chloride. The red liquid was recovered in poor yield by vacuum distillation at —30°C. The product decomposes at —15°C with the evolution of methane and the formation of a black color. When a mole ratio of ZrCl 4 LiCH g( 1 6) is used, the addition of hexane to the ether/toluene solution results in a chloride-free precipitate whose elementary analysis corresponds to the composition Li2[Zr(CH3)g]. This product decomposes at 0°C. 

BRO] Bromberg, M. L., Purification of zirconium tetrachlorides by fractional distillation. Report Patent US2852446, (1958). Cited on page 170. 

Removal of the hafnium tetrachloride from the zirconium tetrachloride cannot easily be achieved in a direct manner. However, the addition compounds with phosphorus pentachloride and with phosphorus oxychloride (3Zr(Hf)Cl4.2POCls) have been separated by fractional distillation on a small scale.A proposal has also been made to separate the chloride by vapour phase dechlorination with a mixture of chlorine and oxygen, making use of the difference in equilibrium constant for the two reactions ... 

The pyrometallurgical methods were developed based on the differences between zirconium and hafnium in oxidation and reduction characteristics [11, 12] volatility [13-16] electrochemical properties [17-19] and molten metal-molten salt equilibrium [20, 21], The extractive distillation process, using carbochlori-nation of zircon [13], is in operation by CEZUS in France. Both chlorides are sublimated and run through a vertical distillation column containing molten aluminium chloride and potassium chloride. Both hafnium and zirconium tetrachloride chlorides dissolve, but hafnium tetrachloride has a higher vapour pressure and is therefore condensed from the top of the column in a hafnium-enriched mixture. The zirconium tetrachloride is partitioned to a liquid phase and recovered from a salt, typically containing less than 50 ppm hafnium. 

In France, Compagnie Europnene du Zirconium (CEZUS) now owned jointly by Pechiney, Eramatome, and Cogema, uses a separation (14) based on the extractive distillation of zirconium—hafnium tetrachlorides in a molten potassium chloride—aluminum trichloride solvent at atmospheric pressure at 350°C. Eor feed, the impure zirconium—hafnium tetrachlorides from the zircon chlorination are first purified by sublimation. The purified tetrachlorides are again sublimed to vapor feed the distillation column containing the solvent salt. Hafnium tetrachloride is recovered in an enriched overhead fraction which is accumulated and reprocessed to pure hafnium tetrachloride. 

Zirconium and hafnium are separated by fractional distillation of the anhydrous tetrachlorides in a continuous molten solvent salt KCl—AlCl system at atmospheric pressure (56,57). Zirconium and hafnium tetrachlorides are soluble in KCl—AlCl without compound formation and are produced simultaneously. 

Willi improved means to separate the compounds of these two elements, future research will yield more details of specific hafnium compounds. The methods of separation used effectively include ion exchange techniques, a particularly effective one using u column of silica gel. w ith a solution of the tetrachlorides in methanol as feed and a 1.9 N HCI solution as eluant for zirconium. Separations also have heen accomplished through the distillation of the phosphorus oxychloride addition products. 

Zircon silicate is the most important source of hafnium. Ion-exchange and solvent-extraction techniques have supplanted fractional crystallization and distillation as the preferred methods of separating hafnium from zirconium. The metal itself is prepared by magnesium reduction of hafnium tetrachloride (the Kroll process), and by the thermal decomposition of tetraiodide (de Boer-van Arkel process). The annual world production of hafnium metal was about 40 tons at the end of the 1980s (Soloveichik... 

The sodium reduction of titanium tetrachloride was actually carried out as early as 1939 in Germany, and about 670 kg was produced by the Deutsche Gold and Silber Scheideanstalt, during the 1939-45 war. The process, now obsolete, involved reduction in a molten bath of 50 per cent sodium chloride and 50 per cent potassium chloride at 800°C in an atmos phere of hydrogen. The reactors consisted of expendable welded sheet-iron cylindrical vessels, 50 cm diameter by 70 cm deep and 2 mm thick. These rested loosely in a stout iron crucible, fitted into a gas-fired furnace. A portable stirrer was used to agitate the reactor contents. Approximately 20 kg batches of titanium were reduced by distilling 85 kg of titanium tetrachloride at a controlled rate into a melt of 15 kg sodium chloride and 15 kg of potassium chloride, covered with a layer of 46 kg of molten sodium. The titanium sank to the bottom of the molten salts, and at the end of the reaction was recovered from the crushed solidified melt by leaching with dilute hydrochloric acid, in a ceramic-lined vessel. It was finally washed in water and dried at a moderate temperature. The same plant was also used for the production of zirconium metal by the sodium reduction of potassium fluorozirconate (KaZrF ]. 

McLaughlin, D.R and Stoltz, R.A. (1988) Zirconium and hafnium tetrachloride separation by extractive distillation with molten zinc chloride lead chloride solvent. US Patent 4737 244. 

Sathiyamoorthy, D., Shetty, S.M., and Bose, D.K. (1999) Pyrochemical separation of zirconium and hafnium tetrachlorides using fused salt extractive distillation process. High Temp. Mater. Process, 18(4), 213. 

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